Ion acceleration by high-power lasers has attracted significant attention in recent years from the scientific community due to its potential applications in different branches of physics and technology. The physical characteristics of accelerated protons, such as high collimation and high particle flux, make them very attractive for applications in controlled nuclear fusion, material science, and hadron therapy.
The physical processes responsible for ion acceleration during laser-matter interaction are understood on a qualitative level. For high laser intensities (I≦1021 W/cm2), the target normal sheath acceleration (TNSA) mechanism has become a well accepted explanation for rear target proton acceleration. It is believed that the incoming laser pulse quickly ionizes the target pushing some of the electrons out of it through the action of the ponderomotive force. A strong electrostatic field (on the order of teravolts per meter, “˜TV/m”) is set up between the expanding electrons and the target, which field ionizes a thin hydrogen-rich layer present at the target's back surface. Subsequently, the protons are accelerated in this electrostatic field. For thicker targets (≧2 μm) a shock wave acceleration mechanism has also been proposed in which a laser acts as a piston driving a flow of ions into the target and launching an electrostatic shock at the front of the target with high Mach number M=vshock/c is about equal 0.2-0.3. Protons, reflected off the shock front may get accelerated to velocities up to vions=2vshock.
Multi-parametric particle-in-cell (PIC) simulation studies of the interaction between a clean (no prepulse present) high-power laser pulse and thin double-layer target have been made. These studies mapped maximum proton energy regions as functions of target electron density and its thickness as well as laser pulse length for different laser intensities and spot sizes. Protons can be accelerated using laser light to the energy range of about a few hundred MeV (e.g. as required for hadron therapy applications where protons with energy 250 MeV can reach any disease site throughout a patient's body). Such acceleration requires a few hundred joules of energy or equivalently several tens of petawatt of power for laser pulse duration Lp˜100 fs. This energy is pumped into a laser pulse, the characteristics of which are provided for a particular target. Currently available lasers, specifically compact table-top systems, operate in the sub-picosecond regime and provide energy on the order of El˜10 J. According to the scaling laws, current table-top lasers may be insufficient to accelerate protons to the energy range of about 200 to 250 MeV. Therefore, there is a need to increase the maximum proton energy, or equivalently the efficiency of energy transfer from the laser pulse into accelerated protons, without necessarily requiring an increase in laser pulse energy.